Hitherto, all spreaders on crawler tracks have been built preferably as compact spreaders with a fixed ballast boom, or as C-frame spreaders with a fixed ballast carrier. Compact spreaders have so far been used for discharge projections between 30 and 60 m.
The typical characteristics of such a machine are shown in FIG. 1, as the state of the art 1. Via a slewing connection, a sub-structure bears a turntable 4, on which a gantry framework 5 is positioned, to which the discharge boom 8 is hinged in a central position. Onto the top, the slewable connecting bridge 13 is mounted. The counter boom 9 is designed as a rearward extension of the turntable 4 with anchoring (guys) connected to the framework 5. On its back end, the counter boom 9 bears the counter weight 10, and next to it the containers for housing the electrical system and hydraulic system. The discharge boom 8 is supported on the turntable 4 by means of one or two hydraulic cylinder(s) 14.
It is essential that the discharge boom 8 be completely carried by the hydraulic cylinder(s) 14. These hydraulic cylinder(s) 14 serve to adjust the dumping height.
This requires large hydraulic cylinders 14. However, at the same time it poses the danger that, in the case of a leakage, the spreader will start to bear on the front, and then tip over to the back. This is less critical in a compact spreader, since in this case the counter boom rests closely above the subgrade. The size of the hydraulic cylinders limits the application of a compact spreader regarding its discharge projection and band width.
Publication DE 35 32 182 A1 discloses a belt spreader consisting of a sub-structure equipped with crawler tracks, with a superstructure mounted onto it using a slewing connection, and having on this superstructure a boom in one part, arranged in a horizontal axis and being designed as a rocker.
On the side where material is received, this boom acts as a conveying boom up to the slewing axis, and on the following side, i.e. in conveying direction, it acts as a discharge boom.
To adjust the boom's incline a hydraulic cylinder is intended. At the free end of the conveying boom, the conveyed material is first transferred to the boom by a tripper car, then conveyed over the whole length of the boom, and finally transferred back at the dumping side of the discharge boom. By thus positioning the boom in the center of gravity no counter weight is required.
C-frame spreaders are used for discharge projections between 50 and 100 m. (in extreme cases for direct dumping up to 200 m). The typical characteristics are shown in FIG. 2. A sub-structure 2 with a 6-crawler tracks carries, via a slewing connection, a turntable 4, which has a gantry framework mounted to its front. On top of this framework, a guyed cable support is mounted, and at its rear, a guyed counter boom 9 is coupled on to it. In lateral view, counter boom 9, framework 5 and turntable 4 are C-shaped. At its rear end, the counter boom 9 bears the counter weight 10, in front of it the hoisting winch for the discharge boom 8, and in front of this it bears the container for the electrical system. The discharge boom 8 having a further cable support is shifted in an articulated manner at the front of the framework 5 and guyed for raising and lowering via a two-cable hoist winch.
An advantage lies here in the fact that there is sufficient height for the upper lashing which limits the forces even in case of great discharge projections. A disadvantage, however, is that the complete superstructure load must be directed in a C-shape around the middle, which must be kept clear for the material transfer. Another disadvantage is that the spreader will tip over to the back in case the cable hoists fail.
Publication DE 84 18 558 U1 discloses a mobile belt spreader on crawler tracks consisting of a sub-structure with a platform mounted on it in a slewing connection. This platform is continued further up as a framework and accommodates the discharge boom in a horizontal slewing axis. At least one hydraulic cylinder, arranged between the platform and the discharge boom, is intended for height adjustment of the discharge boom. On top, the framework accommodates a connecting bridge in a vertical slewing axis. At the bottom, the platform is extended in horizontal direction, leading away from the discharge boom, and encompasses at that point the counter weight and the container for housing the electrical and hydraulic system. In lateral view the platform containing both the framework mounted on top, and the extension below thus appears as a C-shape. At the end intended for material intake, the connecting bridge is supported by a crawler track.
Furthermore, publication DE 1 910 242 B discloses a spreader which also consists, like the equipment described above, of a sub-structure with a platform mounted onto it in a slewing connection, on which a framework is arranged, and just like the superstructure in the first embodiment, it is designed as a C-shape.
It accommodates two slewable discharge booms which are arranged facing each other, with the conveying paths of their respective conveyors running in opposite directions, whereby one conveyor is intended for dumping above work level and the other conveyor for dumping below work level.
In the center of the machine, there is a distribution unit for bulk solids in the transfer area between the conveying belt bridge and the two discharge conveyors. Since the two discharge booms are facing each other, a favourable mass distribution is achieved.
In a second embodiment, the framework of the superstructure is not designed as a C-shape but as a tower. Moreover, these two embodiments differ in that both discharge booms are in one case constructed as consisting of two separate parts, and in the other case, they are constructed having a rigid connection to another, and consisting of a single part.
A slightly different mode of design is known from KURTH: Reihe Fördertechnik, Band Tagebaugroβgeräte and Universalbagger, Verlag Technik Berlin, 1971, page 160, illustr. no 3.129 “Bandabsetzer AsRsB-15400.120”.
Further variants have been built over the course of the last decades. All the C-frame spreaders share the common characteristics, that the forces of the superstructure are transferred in a C-shape around the central transfer area. Furthermore, in all the machines mentioned, the discharge boom 8 is completely supported by the hydraulic cylinders, respectively the cable hoists. In case of failure, this leads to loss of stability.